Automotive air conditioning compressors are generally not directly powered from the drive belt, but are actuated indirectly through an electromagnetic clutch, which allows the compressor to be turned on and off on demand, rather than running continually. A typical clutch includes a friction disk, which is fixed to a belt driven pulley that rotates freely on a ball bearing surrounding and coaxial to the compressor drive shaft. A central hub fixed to the drive shaft mounts an annular ferrous armature in axial opposition to the pulley so that an electromagnetic coil can pull the armature into frictional engagement with the pulley friction disk. The pulley can then transfer rotation and power to the compressor drive shaft. There are numerous existing designs for the component that mounts the armature to the hub, each of which has certain advantages and disadvantages.
It is necessary that the armature mount, of whatever construction, be axially flexible enough to allow the coil to pull the armature into and against the friction disk, but torsionally stiff enough to thereafter transfer rotation to the shaft. Some torsional resilience is desirable, however, to cushion the shock of initial disk-armature engagement, and so as to dampen torsional vibrations in operation. In the radial direction, it is best that the armature support be very stiff, so that the heavy armature will not whirl off axis about the shaft after engagement. It is also an advantage if that portion of the armature support in actual contact with the armature be insulative, so as to not interfere with the electromagnetic circuit therethrough, and it is also an advantage if it allows for a wide variety of armature designs, such as separate, concentric rings, to be easily joined thereto. That portion of the armature support directly connected to the central hub should be durable, and able to take repeated cycling and flexing without failure, since it will be highly stressed. In that regard, some fail safe mechanism to retain the armature support axially to the hub should the main connection fail is sometimes needed. It is helpful if the armature support acts as a cover, to exclude contaminants, and provides acoustic dampening. Finally, simplicity of components, and a minimal number thereof, as well as ease of manufacture and assembly, are always design goals.
Known designs fall short relative to the ideal features listed above. The most common armature support is simply multiple metal leaf springs fixed at their inner ends to the hub, and at their outer ends to the armature. The springs thus provide both a physical mount for the armature as well as the needed axial flex. Such a design is simple and durable, but wanting as to almost every other desirable feature. Separate leaf springs are radially stiff, but have little inherent torsional resilience. They are also typically steel, which is magnetically conductive, and it is difficult to use them for attaching anything but a standard, one piece armature piece to the central hub. Armatures made up of separate rings require much more complex spring designs. Leaf springs, of course, by themselves, provide no real protective cover for the from of the compressor, and are inherently noisy.
Proposed alternatives to the standard, separate leaf spring design generally involve a one piece, molded plastic or rubber disk to replace the separate metal springs. One basic design uses a thin, hard plastic disk, the inner edge of which is molded rigidly fixed to the central hub. The outer edge mounts the armature, which can be either one piece or separate rings molded into the disk. Since the disk is continuous, it inherently provides a good dust and acoustic cover. An example may be seen in U.S. Pat. No. 5,036,964 to Booth et al. Since the plastic material of the disk 45 is fairly rigid, it must be axially thin in order to have an axial flexibility comparable to a metal leaf spring. The potentially weak point of such a design is the integrally molded live hinge juncture between the plastic disk's inner edge and the hub, where all the flexing and engagement stress must be resisted and accommodated. The '964 design attempts to strengthen that juncture as much as possible by molding the inner edge into a wide, cylindrical sleeve 41, and in turn molding that sleeve around multiple lugs 42 extending out from the hub 36. Even with such a hinge, however, one embodiment provides a separate retention plate 80 to retain the disk 45 in case of structural failure. Moreover, the plastic material does not really have enough torsional flexibility to provide a good torque cushion. In fact, stiff, thin plastic disks have been found to be so impractical in terms of fatigue strength that co assigned U.S. Pat. No. 5,377,799 to Mullaney et al proposes to use a thin metal disk in a similar fashion. The metal is strong enough to flex repeatably, provides an integral return spring and dust cover, is quiet, and gives up little in terms of torsional flexibility, as compared to rigid plastics.
Given this inherent shortcoming of the more rigid plastics, it has been proposed to use more flexible, rubber like elastomer material. An example may be seen in U.S. Pat. No. 4,445,606 to Van Laningham. A bifurcated central hub having inner and outer metal sleeves has a radially thin layer of torque cushioning elastomer molded integrally between the sleeves. The outer sleeve is fixed to the drive shaft, while the outer sleeve mounts the armature through metal leaf springs. The amount of torque cushioning that is provided is limited by the radial thinness of the elastomer layer, and, of course, none of the other drawbacks inherent to separate metal leaf springs is eliminated. The more successful approach has been to use a radially wide, axially thick disk of elastomer material to directly mount the armature to the hub. This goes the farthest toward providing all the desirable features noted above. The thick layer of torsionally flexible elastomer provides good torque cushioning, replaces the separate leaf springs, dampens noise, and acts as a dust cover, and is a very simple design. An early example of such a design may be seen in co assigned U.S. Pat. No. 3,384,213 to Bernard et al, in which the elastomer cushion has a uniform axial thickness. An improvement to that basic design may be seen in co assigned U.S. Pat. No. 5,219,273 to Chang, where a unique elastomer pad with an axial thickness that decreases moving radially out is described. The pad is thick enough at the inner edge, where it is mold bonded to the outer surface of the hub, to be durable, and its resilience resists stress and fatiguing, as compared to a more rigid, molded plastic.
An inherent drawback of elastomer, however, as compared to harder plastic disks, is that the extra resilience and flexibility is inevitably reflected in an undesirable radial deformation and buckling. Other co assigned patented designs speak to that inherent problem. In U.S. Pat. No. 5, 195,625 to Chang et al, cylindrical reinforcements are molded into the body of the elastomer cushion to give it added radial stiffness, which at least reduces, but doesn't eliminate, the problem. U.S. Pat. No. 5,390,774 to Thurston et al goes the farthest in eliminating radial whirl of the elastomer cushion, but does not directly stiffen it. Instead, an outer cylindrical steel ring is fixed over the outer edge of the elastomer cushion, while a guide plate welded to the central hub has a cylindrical guide flange that overlaps the outer surface of the outer steel ring. Therefore, the ring can slide axially within the guide flange as the armature and pad are pulled toward the pulley disk, but the close radial confinement of the ring within the flange effectively eliminates radial whirl in the cushion. Still, the addition of the extra ring and plate add complexity and cost, and the radial interfit of the two has to be close enough to give guidance, but not so close as to bind and retard free axial sliding. And, of course, the benefits of being able to solidly mold an armature or separate armature rings into a rigid plastic are not available when using a softer elastomer cushion.